1. Field of the Invention
The present invention relates to an imaging optical system.
2. Description of the Related Art
There is a need for surveillance cameras and vehicle cameras to be compact and for imaging optical systems for such cameras to be smaller since space for mounting is often limited. Furthermore, surveillance cameras and vehicle cameras are often used at night, requiring a bright imaging optical system. In response to such demands, numerous imaging optical systems have been proposed that can be mounted on surveillance cameras, vehicle cameras, etc. (for example, refer to Japanese Patent Application Laid-Open Publication Nos. 2013-92774, 2013-47753, and 2008-8960).
The optical system disclosed in Japanese Patent Application Laid-Open Publication No. 2013-92774 is configured by five lenses and has an F number on the order of 2.4. The optical systems disclosed in Japanese Patent Application Laid-Open Publication Nos. 2013-47753 and 2008-8960 are configured by five lenses and have an F number of 2.0.
In recent years, surveillance cameras and vehicle cameras have come down in price and lower-cost imaging optical systems for such cameras are also demanded. Further, with the rapid increases in the pixel density of solid state image sensors (CCD, CMOS, etc.), bright, high-resolution imaging optical systems capable of supporting solid state image sensors with high pixel densities have come to be demanded.
Surveillance cameras are often installed outdoors, where the temperature varies greatly. Further, vehicle cameras are installed inside vehicles, where especially during the summer, the temperature becomes very high. Thus, among imaging optical systems for surveillance cameras and vehicle cameras, an imaging optical system that can maintain high resolution over a wide temperature range, from low temperatures to high temperatures, is demanded. In particular, an optical system having a deep depth of focus is more advantageous in preventing focus errors during extreme temperatures. Further, if the depth of focus is deep, drops in image quality consequent to shifts in lens centers at the time of product assembly can also be prevented.
The optical system described in Japanese Patent Application Laid-Open Publication No. 2013-92774 is configured by five lenses and therefore, has a low manufacturing cost and is also a bright lens having an F number of 2.4. Nonetheless, resolution is low because various types of aberration are not sufficiently corrected, making high quality images difficult to obtain. In particular, since the depth of focus is shallow, focus errors occur easily with extreme temperatures, making high resolution difficult to maintain over a wide temperature range, from low temperatures to high temperatures. If the depth of focus is shallow, a further problem arises in that image quality is easily affected by shifts in lens centers occurring at the time of product manufacturing.
The optical system described in Japanese Patent Application Laid-Open Publication No. 2013-47753 is also configured by five lenses and therefore, has a low manufacturing cost and is a bright lens having an F number of 2.0. Nonetheless, resolution is low because various types of aberration are not sufficiently corrected, making high quality images difficult to obtain. Further, since the temperature coefficient of refractive index of the glass lens material is large, focus errors during extreme temperatures are large, making high resolution difficult to maintain over a wide temperature range, from low temperatures to high temperatures.
The optical system described in Japanese Patent Application Laid-Open Publication No. 2008-8960 is also configured by five lenses and therefore, has a low manufacturing cost and is a bright lens having an F number of 2.0. Nonetheless, resolution is low because various types of aberration are not sufficiently corrected, making high quality images difficult to obtain. Further, similar to the optical system described in Japanese Patent Application Laid-Open Publication No. 2013-92774, since the depth of focus is shallow, focus error occurs easily with extreme temperatures, making high resolution difficult to maintain over a wide temperature range, from low temperatures to high temperatures. If the depth of focus is shallow, a further problem arises in that image quality is easily affected by shifts in lens centers occurring at the time of product manufacturing.